Scale keyframes for multiple objects with a script
I found a solution, however it resizes keypoints around arbitrary point.

old_type = bpy.context.area.type
bpy.context.area.type = 'GRAPH_EDITOR'
bpy.ops.graph.interpolation_type(type='CONSTANT')
bpy.context.scene.frame_current = 1
bpy.ops.transform.resize(value=(6, 1, 1))
bpy.context.area.type = old_type